Novel analyses of long-term data provide a scientific basis for chlorophyll-a thresholds in San Francisco Bay

Details

• Journal Title:
  Estuarine, Coastal and Shelf Science
• Personal Author:
  Sutula, Martha ; Kudela, Raphael ; Hagy, James D. ; Harding, Lawrence W. ; Senn, David ; Cloern, James E. ; Bricker, Suzanne ; Berg, Gry Mine ; Beck, Marcus
• NOAA Program & Office:
  NCCOS (National Centers for Coastal Ocean Science) ; NOS (National Ocean Service)
• Description:
  San Francisco Bay (SFB), USA, is highly enriched in nitrogen and phosphorus, but has been resistant to the classic symptoms of eutrophication associated with over-production of phytoplankton. Observations in recent years suggest that this resistance may be weakening, shown by: significant increases of chlorophyll-a (chl-a) and decreases of dissolved oxygen (DO), common occurrences of phytoplankton taxa that can form Harmful Algal Blooms (HAB), and algal toxins in water and mussels reaching levels of concern. As a result, managers now ask: what levels of chl-a in SFB constitute tipping points of phytoplankton biomass beyond which water quality will become degraded, requiring significant nutrient reductions to avoid impairments? We analyzed data for DO, phytoplankton species composition, chl-a, and algal toxins to derive quantitative relationships between three indicators (HAB abundance, toxin concentrations, DO) and chl-a. Quantile regressions relating HAB abundance and DO to chl-a were significant, indicating SFB is at increased risk of adverse HAB and low DO levels if chl-a continues to increase. Conditional probability analysis (CPA) showed chl-a of 13 mg m-3 as a "protective" threshold below which probabilities for exceeding alert levels for HAB abundance and toxins were reduced. This threshold was similar to chl-a of 13 - 16 mg m-3 that would meet a SFB-wide 80 % saturation Water Quality Criterion (WQC) for DO. Higher "at risk" chl-a thresholds from 25 - 40 mg m-3 corresponded to 0.5 probability of exceeding alert levels for HAB abundance, and for DO below a WQC of 5.0 mg L-1 designated for lower South Bay (LSB) and South Bay (SB). We submit these thresholds as a basis to assess eutrophication status of SFB and to inform nutrient management actions. This approach is transferrable to other estuaries to derive chl-a thresholds protective against eutrophication.
• Keywords:
  Aquatic Science Oceanography
• Source:
  Estuarine, Coastal and Shelf Science, 197, 107-118
• DOI:
  https://doi.org/10.1016/j.ecss.2017.07.009
• ISSN:
  0272-7714
• Format:
  PDF
• Publisher:
  Elsevier BV
• Document Type:
  Journal Article
• Rights Information:
  Accepted Manuscript
• Compliance:
  Library
• Main Document Checksum:
  urn:sha-512:0f4bfa9319cb45bd97c38446af263a05711303599a7d25c550b62409e831de75add3ea2a09458d87e390a94e04a0c2c7872a0dabbd9ab52c2dec4194c2a42837
• Download URL:
  https://repository.library.noaa.gov/view/noaa/64382/noaa_64382_DS1.pdf
• File Type:
  [PDF - 495.57 KB ]